1. Field of the Invention
The present invention relates to a catalytic converter for effectively cleaning the exhaust gas of an automotive internal combustion engine by removal of nitrogen oxide (NOx), carbon monoxide (CO) and hydrocarbons (HC). The present invention also relates to an oxygen-storing complex oxide which may be advantageously used for such a catalytic converter.
2. Description of the Related Art
As is well known, the exhaust gas of an automotive internal combustion engine inevitably contains harmful substances such as NOx, CO and HC. In recent years, particularly, the restrictions on exhaust gas cleaning are increasingly strict for environmental protection.
A so-called three-way catalytic converter has been most widely used for removing the above-described harmful substances. Typically, a three-way catalytic converter includes a honeycomb support made of a heat-resistant material such as cordierite, and a wash-coat formed on the surfaces of the respective cells of the honeycomb support. The wash-coat contains a heat-resistant inorganic oxide such as Al2O3, a catalytically active substance such as Pt, Pd and/or Rh, and an oxygen-storing oxide such as CeO2. The catalytically active substance reducs NOx to N2 while oxidizing CO and HC to CO2 and H2O, respectively.
The oxygen-storing oxide, typically CeO2, has an oxygen storing capacity (hereafter abbreviated as xe2x80x9cOSCxe2x80x9d); that is, the capacity to occlude gaseous oxygen and to release the occluded oxygen. More specifically, CeO2 is added for adjusting the oxygen concentration of gaseous atmosphere, so that excess oxygen in the gaseous atmosphere is occluded into the crystalline structure of CeO2 in an oxygen-rich state (i.e., fuel-lean state which may be simply referred to as xe2x80x9clean statexe2x80x9d) for assisting the catalytic converter in reducing NOx to N2 while releasing the occluded oxygen into the gaseous atmosphere in a CO- and/or HC-rich state (i.e., fuel-rich state which may be simply referred to as xe2x80x9crich statexe2x80x9d) for assisting the catalytic converter in oxidizing CO and HC to CO2 and H2O. Thus, the catalytic activity of the catalytically active substance is enhanced by the addition of CeO2.
However, it has been found that grains or particles of CeO2 grows due to sintering at high temperature. Such growth of CeO2 results in a decrease of surface area, consequently causing gradual loss of OSC. Particularly, if the catalytic converter is mounted near the engine, it may be frequently subjected to an extremely high temperature of no less than 900xc2x0 C. (or sometimes even higher than 1,000xc2x0 C.), which prompts the grain growth of CeO2.
Further, CeO2 provides its intended OSC only under a condition where an oxidizing atmosphere (corresponding to a lean state) and a reducing atmosphere (corresponding to a rich state) are alternately repeated. More specifically, CeO2 is capable of occluding oxygen only after it has previously undergone a reducing atmosphere for releasing the previously occluded portion of oxygen, whereas it is capable of releasing oxygen only after it has previously undergone an oxidizing state for occluding oxygen. Therefore, the air-fuel mixture supplied to the engine needs to be controlled in a narrow range (referred to as xe2x80x9cwindowxe2x80x9d) near the stoichiometric state such that a lean state and a rich state are alternately repeated.
In view of the above problem, an oxygen sensor may be provided for monitoring the oxygen concentration of the exhaust gas, and the output of the oxygen sensor is used for controlling the air-fuel mixture in the narrow window. However, the oxygen sensor may be deteriorated during operation, so that it is possible that the control center point may unexpectedly shift from the stoichiometric state to a lean side. In such a case, CeO2 in the catalytic converter may be always put in an oxidizing atmosphere and thus continue to occlude oxygen. As a result, CeO2 becomes fully loaded with oxygen and is incapable of releasing it as long as the air-fuel mixture is held at the stoichiometric state or a lean state.
It is, therefore, an object of the present invention to provide a catalytic converter for cleaning exhaust gas which is capable of retaining a high catalytic activity for a long time even under severe operating conditions above 900xc2x0 C.
Another object of the present invention is to provide an oxygen-storing oxide which, when incorporated in a catalytic converter, is capable of effectively storing and releasing oxygen even if the control center point for the air-fuel mixture shifts from the stoichiometric state to a lean side.
According to one aspect of the present invention, a catalytic converter for cleaning exhaust gas comprises a heat-resistant support, and a coating formed on the support, the coating including at least one kind of catalytically active substance and at least one kind of oxygen-storing oxide, wherein the oxygen-storing oxide is selected from oxides of Pr and Tb.
The inventors have found that the an oxide of Pr or Tb exhibits a much higher OSC than CeO2 both before and after performing high-temperature aging. Therefore, a catalytic converter utilizing an oxide of Pr or Tb in place of or in combination with CeO2 is capable of providing a high catalytic activity over a long period even under a severe high-temperature operating condition.
In a first embodiment of the present invention, the oxygen-storing oxide is Pr6O11. Such a simple oxide of Pr may be used in combination with CeO2 or Cexe2x80x94Zr complex oxide.
In a second embodiment of the present invention, the oxygen-storing oxide is Tb4O7. Again, such a simple oxide of Tb may be used in combination with CeO2 or Cexe2x80x94Zr complex oxide.
In a third embodiment, the oxygen-storing oxide is a complex oxide of the following formula,
Ce1xe2x88x92(x+y)RxEyOxide
where xe2x80x9cRxe2x80x9d represents Pr or Tb, xe2x80x9cExe2x80x9d represents at least one element selected from a group consisting of Nd, Y, Gd and Zr, 0.1xe2x89xa6xxe2x89xa60.8, 0xe2x89xa6yxe2x89xa60.9, and 0.1xe2x89xa6x+yxe2x89xa60.9. It should be appreciated that the notation xe2x80x9cOxidexe2x80x9d is used because the proportion of oxygen in the complex oxide varies depending on the condition of the atmosphere and the valency of the co-existing elements other than Ce.
As previously described, CeO2 provides an intended OSC only under a condition where an oxidizing atmosphere (corresponding to a lean state) and a reducing atmosphere are alternately repeated, consequently necessitating the air-fuel mixture to be controlled in a narrow window range across the stoichiometric state. On the other hand, the inventors have experimentally found that an oxide of Pr or Tb is capable of releasing oxygen not only in a reducing atmosphere but also in an inert atmosphere (corresponding to the stoichiometric state) after it has occluded oxygen in an oxidizing atmosphere.
According to the third embodiment, CeO2 is complexed with an oxide of Pr or Tb. Therefore, the CeO2 portion of the resulting complex oxide provides a good OSC under a condition where an oxidizing atmosphere and a reducing atmosphere are alternately repeated, whereas the Pr or Tb oxide portion of the complex oxide provides a good OSC under a condition where an oxidizing atmosphere and an inert atmosphere are alternately repeated. Thus, even if the control center point of an oxygen sensor shifts from the stoichiometric state to a lean side due to a deterioration, the oxygen-storing complex oxide can still provide a good OSC.
Further, any one of Nd, Y, Gd and Zr which may be added to the oxygen-storing complex oxide restrains grain growth of the complex oxide under high temperature. Thus, the catalytic converter incorporating the oxygen-storing complex oxide is capable of retaining a high catalytic activity for a long time even under a severe high-temperature operating condition. However, it is also possible to dispense with such a grain growth restraining element.
In the case where the xe2x80x9cExe2x80x9d in the above formula is selected from a group consisting of Nd, Y and Gd, the complex oxide may preferably meet the relations 0.2xe2x89xa6xxe2x89xa60.6, 0.05xe2x89xa6yxe2x89xa60.1, and 0.25xe2x89xa6x+yxe2x89xa60.7. On the other hand, in the case wherein the xe2x80x9cExe2x80x9d in the formula is Zr, the complex oxide should preferably meet the relations 0.2xe2x89xa6xxe2x89xa60.6, 0.2xe2x89xa6yxe2x89xa60.6, and 0.4xe2x89xa6x+yxe2x89xa60.8. Further, at least part of the complex oxide may preferably be solid solution.
Typicaly, the catalytically active substance may be a precious metal such as Ru, Rh, Pd, Ag, Os, Ir, Pt and Au. Preferably, however, the catalytically active substance may be selected from a group consisting of Pt, Rh and Pd. Each of these active substances may be used alone or in combination with another.
The coating may further contains at least one heat-resistant inorganic oxide selected from a group consisting of alumina, silica, titania, magnesia and ziroconia. Particularly useful is activated alumina.
The heat-resistant support, which may be made of cordierite, mullite, xcex1-alumina or a metal (e.g. stainless steel), should preferably have a honeycomb structure.
The coating may have a single layer structure. Alternatively, the coating may comprise a plurality of laminated layers including an outermost layer, and the oxygen-storing oxide may be contained in at least one of the coating layers. In the latter, preferably, Pd and Rh as the catalytically active substances should be separately contained in different layers of the coating because the co-existence of these elements may adversely affect each other in catalytic activity. Further, Pd should preferably be contained in a layer of the coating other than the outermost layer because Pd is liable to suffer catalytic poisoning.
According to a second aspect of the present invention, there is provided an oxygen-storing complex oxide for a catalytic converter having the following formula,
Ce1xe2x88x92(x+y)RxEyOxide
where xe2x80x9cRxe2x80x9d represents Pr or Tb, xe2x80x9cExe2x80x9d represents at least one element selected from a group consisting of Nd, Y, Gd and Zr, 0.1xe2x89xa6xxe2x89xa60.8, 0xe2x89xa6yxe2x89xa60.8, and 0.1xe2x89xa6x+yxe2x89xa60.9.
The oxygen-storing complex oxide having the above formula may be prepared by using known techniques such as coprecipitation process or alkoxide process.
The coprecipitation process includes the steps of preparing a mixture solution which contains respective salts of Ce, Pr (or Tb) and Nd (or Y or Gd or Zr) in a predetermined stoichiometric ratio, then adding an aqueous alkaline solution or an organic acid to the salt solution for causing the respective salts to coprecipitate, and thereafter heat-treating the resulting coprecipitate for oxidization to provide a target complex oxide.
Examples of salts of Ce, Pr (or Tb) and Nd (or Y or Gd) include sulfates, nitrates, hydrochlorides, phosphates, acetates and oxalates. Examples of Zr salts include oxychloride, oxynitrate, oxysulfate and zirconium oxyacetate. Examples of aqueous alkaline solutions include an aqueous solution of sodium carbonate, aqueous ammonia and an aqueous solution of ammonium carbonate. Examples of organic acids include oxalic acid and citric acid.
The heat treatment in the coprecipitation process includes a heat-drying step for drying the coprecipitate at about 50xcx9c200xc2x0 C. for about 1xcx9c48 hours after filtration, and a baking step for baking the coprecipitate at about 350xcx9c1,000xc2x0 C. (preferably about 400xcx9c700xc2x0 C.) for about 1xcx9c12 hours. During the baking step, the baking conditions (the baking temperature and the baking period) should be selected depending on the composition of the oxygen-storing complex oxide so that at least part of the complex oxide is in the form of solid solution.
The alkoxide process includes the steps of preparing an alkoxide mixture solution which contains Ce, Pr (or Tb) and Nd (or Y or Gd or Zr) in a predetermined stoichiometric ratio, then adding a deionized water to the alkoxide mixture solution or causing Ce, Pr (or Tb) and Nd (or Y or Gd or Zr) to hydrolyze, and thereafter heat-treating the resulting hydrolysate to provide a target complex oxide.
Examples of alkoxides usable for preparing the alkoxide mixture solution include respective methoxides, ethoxides, propoxides and butoxides of Ce, Pr (or Tb) and Nd (or Y or Gd or Zr). Instead, ethylene oxide addition salts of each of these elements are also usable.
The heat treatment in the alkoxide process may be performed in the same way as that in the coprecipitation process.
A precious metal such as Pt, Rh or Pd as a catalytically active substance may be supported on the oxygen-storing complex oxide by using known techniques. For instance, a solution containing a respective salt (e.g. 1-20 wt %) of Pt (and/or Rh and/or Pd) is first prepared, the complex oxide is then impregnated with the salt-containing solution, and thereafter the complex oxide is heat-treated. Examples of salts usable for this purpose include nitrate, dinitro diammine nitrate, and chloride. The heat-treatment, which is performed after impregnation and filtration, may include drying the complex oxide by heating at about 50xcx9c200xc2x0 C. for about 1xcx9c48 hours and thereafter baking the complex oxide at about 350xcx9c1,000xc2x0 C. for about 1xcx9c12 hours.
Alternatively, a precious metal may be supported on the oxygen-storing oxide at the time of performing the coprecipitation process or the alkoxide process by adding a salt solution of the precious metal to the mixture salt solution or the alkoxide mixture solution.
Other features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments given with reference to the accompanying drawings.